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似乎通过使用成功测试命名管道的存在会os.path.exists()阻止管道工作。为什么会这样？

这是成功工作的 Windows 命名管道代码：

import time
import multiprocessing as mp
import win32pipe, win32file

PIPENAME = r'\\.\pipe\Foo'

def producer(pipe_name: str):
    print('producer')
#    if not os.path.exists(pipe_name):
#        print(f'No pipe {pipe_name}')
#        return
    pipe = win32file.CreateFile(pipe_name,
                                win32file.GENERIC_READ | win32file.GENERIC_WRITE,  # dwDesiredAccess
                                0,  # dwShareMode
                                None,  # lpSecurityAttributes
                                win32file.OPEN_EXISTING,  # dwCreationDisposition
                                0,  # dwFlagsAndAttributes
                                None
                                )
    win32pipe.SetNamedPipeHandleState(pipe, win32pipe.PIPE_READMODE_MESSAGE, None, None)
    win32file.WriteFile(pipe, b'foobar')


def receiver(pipe_name: str):
    print('receiver')
    pipe = win32pipe.CreateNamedPipe(pipe_name,
                                     win32pipe.PIPE_ACCESS_DUPLEX,
                                     win32pipe.PIPE_TYPE_MESSAGE | win32pipe.PIPE_READMODE_MESSAGE | win32pipe.PIPE_WAIT,
                                     1,  # nMaxInstances
                                     65536,  # nOutBufferSize
                                     65536,  # nInBufferSize
                                     0, # 50ms timeout (the default)
                                     None) # securityAttributes
    win32pipe.ConnectNamedPipe(pipe)
    msg = win32file.ReadFile(pipe, 65536)
    print(f'msg: {msg}')

if __name__ == '__main__':
    recv_p = mp.Process(target=receiver, args=(PIPENAME,))
    prod_p = mp.Process(target=producer, args=(PIPENAME,))
    recv_p.start()
    time.sleep(0.1)
    prod_p.start()
    prod_p.join()
    recv_p.join()

这按预期工作，接收者打印接收到的消息。

但是如果生产者中的三个注释掉的行没有注释，os.path.exists(pipe_name)调用会以某种方式破坏管道，因此输出变为：

receiver
producer
Process Process-2:
Process Process-1:
Traceback (most recent call last):
  File "C:\Users\redacted\AppData\Local\Programs\Python\Python37\lib\multiprocessing\process.py", line 297, in _bootstrap
    self.run()
  File "C:\Users\redacted\AppData\Local\Programs\Python\Python37\lib\multiprocessing\process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "C:\git\redacted\named_pipe_mqtt_test.py", line 18, in producer
    None
pywintypes.error: (231, 'CreateFile', 'All pipe instances are busy.')
Traceback (most recent call last):
  File "C:\Users\redacted\AppData\Local\Programs\Python\Python37\lib\multiprocessing\process.py", line 297, in _bootstrap
    self.run()
  File "C:\Users\redacted\AppData\Local\Programs\Python\Python37\lib\multiprocessing\process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "C:\git\redacted\named_pipe_mqtt_test.py", line 35, in receiver
    msg = win32file.ReadFile(pipe, 65536)
pywintypes.error: (109, 'ReadFile', 'The pipe has been ended.')

为什么会os.path.exists破坏命名管道的窗口？

我已经排除了 python 多处理库。之后我尝试过延迟os.path.exists。

这对我来说不是阻塞问题，但我很好奇。

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1 回答 1

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os.path.exists委托给os.stat，并在 Windows 上os.stat尝试打开文件：

hFile = CreateFileW(path, access, 0, NULL, OPEN_EXISTING, flags, NULL);

然后它关闭文件。此时，除非服务器端首先调用DisconnectNamedPipe，否则客户端无法重新打开管道，这在此代码中不会发生。此外，当服务器端尝试从管道中读取数据时，它正在从os.stat的连接尝试中读取，这会关闭管道而不写入任何数据。

我会认为这是一个错误。os.stat应该不会有这样的副作用。

于 2021-04-15T05:48:11.190 回答